"Lord of War" traces the story of a big-time arms dealer over the course of 18 years, but it plays more as an excuse for writer-director Andrew Niccol to investigate the general topic of arms dealing rather than an effort to tell a specific story. It's like a subject in search of a drama. The film is always at least mildly interesting, because international arms dealing is a fairly compelling issue, but it's never as informative as a good documentary nor as engrossing as a good narrative. It's a hybrid that's frustrating in two distinct ways.

The missed dramatic opportunities make for the first frustration. Nicolas Cage plays Yuri, a Ukrainian American from Brooklyn, who's living a go-nowhere life, when he witnesses a gangland assassination close-up. His immediate reaction to seeing two guys get killed in front of him isn't to get scared or want to move to a better neighborhood. Rather, in a flash, he decides he wants to sell illegal weapons, figuring that the market will never dry up, that people will always want to kill each other.

One would think that any fellow who's that crazy, amoral and adventurous deserves to be the subject of a film, and who better than Cage to play such a combination of bizarre inspiration and cold calculation? But soon we realize that Yuri's arms-dealing epiphany is not there for the purpose of introducing us to a wild personality but rather as a kind of shorthand, to get our hero into the gun business as quickly as possible. That Yuri is played by Cage is a calculated distraction: Cage's previous association with thoughtful but volatile characters helps to disguise, for a time, that Yuri is in fact a cipher, a sober and bland figure whose internal life remains a mystery, not because he's mysterious, but because nothing is there.

To put it in another way, Yuri is not dead inside -- rather, he wasn't given insides by the writer. He's a shell, perhaps a composite, meant to stand as an exemplar of all arms dealers. This strategy might have worked in dramatic terms, if Yuri were meant to be seen at a remove, as a dark, elusive figure haunting and helping to create scenes of human misery. But "Lord of War" is structured more like a classic tragedy, in which a capable but overly ambitious man succumbs to temptation, achieves great heights but loses his soul. The catch is that a character has to have a soul in order to lose one.

To fill out the human side of Yuri, Niccol attempts to ground him in relationships. He is given a brother (Jared Leto), with a substance abuse problem that has nothing to do with anything. He is given old-world parents. His wife is a potentially interesting character, especially as played by Bridget Moynahan -- she's a former model with artistic longings and a strong moral core. But though she and Yuri are presented as a love match, their relationship is way in the background and doesn't exactly make sense. So far as we can tell, he sees her every month or so.

In the foreground are Yuri's arms deals, which get bigger and bigger. In the manner of a Scorsese gangster, Yuri narrates his adventures, which take him from Little Odessa in Brooklyn to Eastern Europe and to the Middle East and Africa, but mainly Ukraine and Liberia. In Ukraine, there are unbelievable stockpiles of weaponry left sitting there after the Cold War, and in Liberia there's a charismatic and sadistic despot (Eamonn Walker) with an unlimited supply of enemies.

Along the way, the movie comes up with perfunctory turning points for Yuri, so that we can see him hitting rock bottom morally. But he hits rock bottom more than once, and the second time doesn't have much meaning. Anyway, since he starts the movie just a few inches from the bottom, he can't land with any impact. The audience is barely interested, and Niccol can barely fake it.

The movie's second big frustration is that it inspires questions that it doesn't answer -- that it obviously could not answer, without breaking into straight documentary mode. Just one example of many: Our hero walks into a country and orders a fleet of tanks and assorted helicopter gun ships. He arrives by himself. He's a one-man business. So how does he move that weaponry, since he can't fit it in the overhead compartment? And where does he store it, before he finds a buyer? And how does he pay for it -- Visa? MasterCard? If he shows them his driver's license, will they take a check?

There are the makings here of a really good feature or an even better documentary, but as it stands, "Lord of War" is not quite either.